qt-everywhere-src-5.14.1/qtdoc/doc/src/platforms/atomic-operations.qdoc

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/*!
    \page atomic-operations.html
    \title Implementing Atomic Operations
    \brief A guide to implementing atomic operations on new architectures.

    \ingroup best-practices
    \ingroup qt-embedded-linux

    Qt uses an optimization called \l {Implicitly Shared
    Classes}{implicit sharing} for many of its value classes.

    Starting with Qt 4, all of Qt's implicitly shared classes can
    safely be copied across threads like any other value classes,
    i.e., they are fully \l {Reentrancy and Thread-Safety}{reentrant}.
    This is accomplished by implementing reference counting
    operations using atomic hardware instructions on all the
    different platforms supported by Qt.

    To support a new architecture, it is important to ensure that
    these platform-specific atomic operations are implemented in a
    corresponding header file (\c qatomic_ARCH.h), and that this file
    is located in Qt's \c src/corelib/arch directory. For example, the
    Intel 80386 implementation is located in \c
    src/corelib/arch/qatomic_i386.h.

    Currently, Qt provides two classes providing several atomic
    operations, QAtomicInt and QAtomicPointer. These classes inherit
    from QBasicAtomicInt and QBasicAtomicPointer, respectively.

    When porting Qt to a new architecture, the QBasicAtomicInt and
    QBasicAtomicPointer classes must be implemented, \e not QAtomicInt
    and QAtomicPointer. The former classes do not have constructors,
    which makes them POD (plain-old-data). Both classes only have a
    single member variable called \c _q_value, which stores the
    value. This is the value that all of the atomic operations read
    and modify.

    All of the member functions mentioned in the QAtomicInt and
    QAtomicPointer API documentation must be implemented. Note that
    these the implementations of the atomic operations in these
    classes must be atomic with respect to both interrupts and
    multiple processors.

    \warning The QBasicAtomicInt and QBasicAtomicPointer classes
    mentioned in this document are used internally by Qt and are not
    part of the public API. They may change in future versions of
    Qt. The purpose of this document is to aid people interested in
    porting Qt to a new architecture.
*/